Collective Quantum Thermodynamics

Heat engines are the motors of our industrialised society. By converting thermal energy into mechanical work, they set cars, airplanes and ships in motion and drive the generators that deliver electricity to our computers and smartphones. None of these modern applications would be possible without one fundamental theory that emerged 200 years ago and has ever since enabled engineers to develop more and more powerful and efficient machines: thermodynamics. Equipped with only a few elementary concepts and laws, this theory lays down the basic rules that govern the performance of James Watt's 18th century steam engine and today's car engines alike. During the past two decades, a new era has begun, in which scientists are exploring miniaturisation as a novel design principle for thermal engines. In a series of landmark experiments, smaller and smaller engines have been built and successfully operated. In 2016, this fascinating development led to the realisation of a functional heat engine with only one atom. Objects this tiny are no longer bound by the mechanical rules of our classical world; they can occupy two places at the same time, tunnel through barriers or influence each other at a distance without direct interaction. These counterintuitive phenomena are manifestations of the quantum laws of motion that govern the world at atomic scales. Heat engines operating in this realm can be equipped with features that no classical engineer could have imagined. The scientific discipline that describes this new type of machine and tries to harness their technological potential is still in its infancy and has been dubbed quantum thermodynamics. Although likely able to overcome classical performance limits, quantum engines are still far from practical applications, not least due to their minuscule energy output; to move a car, one would need roughly as many single-atom engines as there are molecules in one liter of water. This number is absurdly large, mainly because it compares objects at radically different scales. Still, it is clear that, even to be useful for technologies on their own scale, quantum engines need to grow. But how can their size be increased when smallness is precisely the property that makes them quantum? Quantum mechanics provides a solution to this dilemma: collective behaviour. Due to a strange interaction without a classical counterpart, objects like atoms can act in a coordinated way, like birds in a flock. This remarkable phenomenon has fascinated scientist for decades. Here, we propose to utilise it for the next generation of quantum machines. Imagine an engine working with a collective quantum gas containing millions of atoms instead of just one. Such a device could benefit from quantum effects while still producing significant power output. Moreover, the pistons of this engine could be perfectly synchronized with all the atoms they move around. Thus, an enormous level of control could be achieved, which would be impossible to realise with an ordinary gas, whose atoms follow unpredictable trajectories. Such unique features make collective quantum machines a fascinating yet unexplored subject of quantum engineering. Laying down the conceptual foundations for the design and implementation of this new type of device is the major goal of this project. The theory we will develop at the University of Nottingham will be the counterpart of thermodynamics in the world of collective quantum phenomena: collective quantum thermodynamics. Quantum technologies are widely expected to shape our century in a similar way as the industrial revolution changed 19th and 20th century. Collective quantum machines have the potential to become the steam engines of this development. They will not move our future cars, but they might well provide the power for our quantum computers and encryption devices.